1. Field of the Invention
The present invention generally relates to a brushless motor. More specifically, the present invention pertains to a brushless motor with an improved bearing for supporting the rotor drive shaft.
2. Description of the Related Art
Brushless motors do not require brush replacement and are relatively compact. Therefore, they are used in many applications where space is limited such as for the fan motor in a temperature sensor provided in a sections of vehicles which are hard to access.
Such a brushless motor is disclosed, for example, in Japanese Unexamined Patent Publication Nos. Sho 60-91853 and Sho 61-54860. This brushless motor has a motor housing 32 formed by an upper case 30 and a lower case 31 made of a magnetic material as illustrated in FIG. 3. A rotor 34 is provided in the housing 32. A pair of magnets 33 are symmetrically arranged about a drive shaft 36 at a position below the rotor 34. The magnets 33 are normally attracted by the lower case 31, therefore, when the rotor 34 is at rest, it is held at a stable position. Actuating coils 35 are arranged in the lower case 31 to face the magnets 33. To drive the motor, the actuating coils 35 are sequentially excited to rotate the rotor 34 and the drive shaft 36.
The structure of the bearing of the drive shaft 36 will now be explained. A downward extending recess 37 is formed in the center of the lower case 31, and a resin plate 38 is set against the internal wall of the lower case 31. The lower end portion of the drive shaft 36 is supported in the recess 37 by a radial bushing 39 that sits on the resin plate 38.
A bore 42 is formed in the center of the upper case 30. An anchor nut 43 is press fit in the bore 42. An adjust bolt 44 is screwed into the anchor nut 43 and extends into the housing 32. The bolt's lower end is secured onto the upper surface of a box-shaped support case 40. The support case 40 has a thrust bushing 41 disposed therein. The thrust bushing 41 is configured to provide tiny clearances 45. Supported by the bushing 41 is the upper portion of the drive shaft 36 which is inserted from the bottom of the support case 40. As the length of the engagement of the adjust bolt 44 in the anchor nut 43 varies, the relative positions of the clearances 45 to the bushing 41 in the support case 40 adjusts the play of the drive shaft 36 in the direction of thrust.
In a compact motor outputting a small torque, however, the friction between the drive shaft 36 and the bushing 41 causes the rotor 4 to rotate at a lower velocity than the desired velocity. In order to keep this velocity differential at a minimum, tiny clearances are formed to avoid a large amount of play in the drive shaft 36 in the thrust direction.
However, as the vibration of the vehicle is transmitted to the motor, the clearance between the bushing 41 and the shaft 36 cause the position of the drive shaft 36 to fluctuate gradually against the attracting force between the magnet 33 and lower case 31. Accordingly, the upper end of the drive shaft 36 hits the bushing 41, thus causing noise.